Picked up a pair of these the other day when I left my old fins on the deck and they disappeared. I was impressed with the ability to keep the kicking cadence inline with "normal" vs a fin that slows down the overall beat (thus messing with your stroke)...

I did find that you can certainly size down as i am an 11.5 and easily got into the "L" and had a bit of toe room (the toe box is wide)